1. Field of the Invention
The present invention generally relates to a method for creating mask pattern data for fabricating a circuit for correcting an original mask pattern by optical proximity correction to create corrected mask pattern data, and a method for verifying a mask pattern for fabricating a circuit for verifying that the corrected mask pattern data has been properly corrected. Specifically, the present invention relates to such a method for creation and such a method for verification used for transferring a layout pattern of a large scale integrated circuit with high fidelity by exposing a corrected mask on a wafer.
2. Description of the Related Art
Recently, large scale integrated circuits (LSIs) are increasingly miniaturized, and the layout patterns of the LSIs are increasingly miniaturized. This also requires the photomask patterns used in lithography in an LSI fabrication process to be miniaturized.
When a photomask pattern is extremely miniaturized, it may be difficult to control the size of the photomask pattern or the photomask pattern may be deformed.
One of the reasons for the above problems is optical proximity, which occurs when a pattern is made in a mask. When this occurs, the mask pattern is not reproduced with high fidelity. Another reason is pattern distortion, which occurs when the mask pattern is transferred onto a wafer. When this occurs, the mask pattern is not reproduced with high fidelity.
Conventionally, a light beam having a relatively short wavelength (about 365 nm) is used for exposure. Such a light beam is referred to as an “i beam”. Use of the i beam allows an LSI circuit using a mask pattern having each side of about 0.5 μm to 0.3 μm to be fabricated with a precision of about 0.05 μm. Today, a KrF excimer laser beam having a shorter wavelength (about 248 nm) is used for exposure in a lithography step.
A mask having patterns at a high density is only transferred onto a wafer with a low level of reproducibility. Particularly, a mask having a pattern smaller than the wavelength of light involves the problems described below with reference to FIGS. 8 through 10.
FIG. 8 shows an example of a mask pattern to be exposed and a mask pattern transferred onto a wafer. Reference numeral 101 represents a rectangular mask pattern to be exposed (for example, a pattern for a conductive line), and reference numeral 102 represents a mask pattern transferred onto a wafer. The corners of the mask pattern 102 are rounded by optical proximity, resulting in having portions 103 missing. Consequently, the mask pattern 102 is shorter than the mask pattern 101. This causes electrical disadvantages (for example, a reduction in current capacitance).
FIG. 9 shows another example of a mask pattern to be exposed and a mask pattern transferred onto a wafer. Reference numeral 111 represents a square mask pattern to be exposed (for example, a pattern for a contact hole), and reference numeral 112 represents a mask pattern transferred onto a wafer. The corners of the mask pattern 112 are rounded by optical proximity, resulting in having portions 113 missing.
FIG. 10 shows still another example of a mask pattern to be exposed and a mask pattern transferred onto a wafer. Reference numeral 121 represents a plurality of square mask pattern elements to be exposed, and reference numeral 121′ also represents a mask pattern element to be exposed. The mask pattern elements 121 are arranged regularly at a high density, and the mask pattern element 121′ is located away from the plurality of square mask pattern elements 121. The mask pattern elements 121 and 121′ each have sides having a length “a”.
Reference numeral 122 represents a plurality of mask pattern elements transferred onto a wafer, and reference numeral 122′ also represents a mask pattern element transferred onto the wafer. The corners of the mask pattern elements 122 and 122′ are rounded by optical proximity, resulting in having portion elements 123 and 123′ missing. In such an arrangement, the mask patterns 122 and 122′ have different sizes. For example, each side of one mask pattern element 122 has a length “c”, and each side of another mask pattern element 122 has a length “d”. Each side of the mask pattern element 122′ has a length
This has adverse influences on the operating timings, production yield, and the like of LSI circuits.
The above-described problems caused by optical proximity occur even when light of a short wavelength is used for lithography, and can be solved by correcting, for example, the size or shape of the mask pattern. This is realized by predicting how the mask pattern will be deformed or distorted by optical proximity when transferred onto the wafer.
Such a correction is referred to as “optical proximity correction (OPC)”. A mask processed with OPC is referred to as an “OPC mask”. Especially when miniaturized mask patterns having a design rule (minimum processing size) of 0.35 μm or less is required, OPC and OPC masks are widely used.
Such a correction of mask patterns is conventionally performed based on experience on the size or arrangement of the patterns. As the mask pattern design simulation technology is developed, the mask patterns are now corrected systematically as a part of the LSI circuit design system.
The pattern distortion caused by optical proximity (hereinafter, referred to as a “proximity distortion”) is corrected by OPC as follows. Based on data empirically obtained by exposing test patterns for characteristic evaluation, the proximity distortion is mathematically described using OPC software. Specifically, the mathematic description of the proximity distortion is performed by a technique called “rule-based OPC”. Such a mathematical description of the proximity distortion represents a rule indicating how the layout pattern of the mask is to be changed (correction rule). Based on the rule, a rule set for processing the mask pattern by OPC is created. The mask pattern is processed by OPC in accordance with the rule set.
Alternatively, the mathematical description of the proximity distortion may be performed by a technique called “model-based OPC”. In this case, optical simulation is performed based on design data. According to this technique, the mathematical description of the proximity distortion represents a model indicating how the mask pattern is to be changed (correction model). Based on the model, a model set for processing the mask pattern by OPC is created. The mask pattern is processed by OPC in accordance with the model set. The “model-based OPC” considers optical distortion or process-related distortion predicted to occur when the pattern is transferred onto a wafer, and can cope with more complicated processes.
The OPC software including the rule set or the model set automatically performs correction processing (for example, change of mask patterns, movement of the edges of lines, addition of special patterns, etc.). The correction is performed on data representing a mask pattern which is predicted to be distorted when transferred onto a wafer (for example, the mask pattern 111 in FIG. 9). Thus, corrected mask pattern data is created.
A pattern obtained on a wafer through a mask pattern corrected by OPC reproduces a pattern represented by the design data at higher fidelity than a pattern obtained on a wafer through an uncorrected mask pattern.
The conventional OPC described above is time-consuming, since it is necessary to correct data representing a miniaturized mask pattern and create data representing a miniaturized corrected mask pattern.
FIG. 11 shows an example of a mask pattern corrected by OPC. Each corner of the square mask pattern (for example, a pattern for a contact hole) is provided with a small projection pattern 104. Owing to this, the degree of proximity distortion caused when the mask pattern is transferred onto a wafer is reduced. A pattern of a shape like the projection pattern 104 is referred to as a “serif pattern”.
A square mask pattern as shown in FIG. 11 is corrected into a pattern including 9 quadrangular portions or having 20 corners. Such a correction which increases the number of quadrangular portions requires a long processing time.
FIG. 12 shows another example of a mask pattern corrected by OPC. Each end of the long rectangular mask pattern (for example, a pattern for a conductive line) is provided with a projection pattern 105. Owing to this, the degree of proximity distortion caused when the mask pattern is transferred onto a wafer is reduced. A pattern of a shape like the projection pattern 105 is referred to as a “hammer head”.
A rectangular mask pattern as shown in FIG. 12 is corrected into a pattern including 7 rectangular portions or having 12 corners. Such a correction which increases the number of rectangular portions requires a long processing time.
FIG. 13 shows still another example of a mask pattern corrected by OPC. A projecting corner of the L-shaped mask pattern (for example, a pattern for a projecting corner of a conductive line) is provided with a projection pattern 106, and a recessed corner of the L-shaped mask pattern (for example, a pattern for a recessed corner of a conductive line) is provided with a recessed pattern 107. Owing to this, the degree of proximity distortion caused when the mask pattern is transferred onto a wafer is reduced. A pattern of a shape like the projection pattern 106 is referred to as an “out-corner serif pattern”, and a pattern of a shape like the recessed pattern 107 is referred to as an “in-corner serif pattern”. In this case also, the number of rectangular portions is increased, which requires a long processing time.
As described above with reference to FIGS. 11 through 13, a correction by OPC increases the number of quadrangular portions of a mask pattern as compared to that of the mask pattern represented by the design data. Thus, a long processing time is required.
When the OPC processing program has errors, corrected mask pattern data which should not be created may be created, or corrected mask pattern data which cannot be realized by the production process of the mask may be created.
Japanese Laid-Open Publication No. 11-174659, for example, discloses a verification method (resize check) for verifying that the corrected mask pattern has been properly corrected. This method will be described below.
Oversized mask pattern data and undersized mask pattern data are created. The oversized mask pattern data is created by oversizing the original mask pattern data by a maximum bias. The undersized mask pattern data is created by undersizing original mask pattern data by the maximum bias. The maximum bias is a maximum width by which an edge portion of the line can be corrected by OPC.
The corrected mask pattern data is compared with the oversized mask pattern data and the undersized mask pattern data. When the corrected width of the corrected mask pattern does not exceed the maximum bias, it is determined that “the corrected mask pattern has been properly corrected”.
FIG. 14 shows a procedure of the corrected mask pattern verification method disclosed in Japanese Laid-Open Publication No. 11-174659. The method will be described with reference to FIG. 14.
Step S101: A simple rule is extracted based on empirical data obtained from a result of exposure of a test pattern for characteristic evaluation. The rule is extracted for the purpose of changing the mask pattern. After the rule is extracted, the processing goes to step S102.
Step S102: The optimum correction amount for OPC (maximum bias) is obtained. Then, the processing goes to step S103.
Step S103: A rule file is created based on the extracted rule (step S101) and the optimum correction amount (step S102). Then, the processing goes to step S105.
Step S104: Original mask pattern data which is design data of the mask pattern is created. Then, the processing goes to step S105.
Step S105: An OPC rule set is created based on the rule file (step S103) and the original mask pattern data (step S104). Then, the processing goes to steps S106 and S107.
Step S106: The original mask pattern is oversized by the maximum bias so as to create oversized mask pattern data. The original mask pattern is also undersized by the maximum bias so as to create undersized mask pattern data. Then, the processing goes to step S110.
Step S107: The original mask pattern is divided into a plurality of regions (template size processing). This is performed for the purpose of alleviating the load of the OPC processing. Then, the processing goes to step S108.
Step S108: The plurality of divided regions (templates) are each processed by OPC in accordance with the OPC rule set (step S105). Then, the processing goes to step S109.
Step S109: Corrected mask pattern data is created. Then, the processing goes to step S110.
Step S110: The corrected mask pattern data (step S109), and the oversized mask pattern data and undersized mask pattern data created in step S106, are subjected to subtraction by graphic operation processing, such that data representing the common graphic pattern is deleted, thus comparing the two types of data. Then, the processing goes to step S111.
Step S111: Based on the comparison result, comparison data is created. Then, the processing goes to step S112.
Step S112: A resize check is performed to determine whether or not the created comparison data includes data exceeding the maximum bias. When data exceeding the maximum bias is present, the processing goes to step S113. When data exceeding the maximum bias is not present, it is determined that the corrected mask pattern data has been properly corrected. Thus, the processing goes to step S114.
Step S113: The data exceeding the maximum bias is corrected, so as to create properly corrected mask pattern data. Then, the processing goes to step S114.
Step S114: The properly corrected mask pattern data is output as mask data. Then, the processing goes to step S115.
Step S115: A mask is produced based on the mask data (step S114).
The above-described conventional verification method has the following problems. Unless both of the difference between the original mask pattern and the oversized mask pattern, and the difference between the original mask pattern and the undersized mask pattern, exceed the maximum bias, it cannot be accurately checked whether or not the corrected mask pattern has been properly corrected in accordance with the correction rule or correction model.
In addition, with the conventional verification method, it is required to use different methods for different types of corrected mask pattern data. For example, only one type of rule-based OPC mask pattern data is created, whereas a plurality of types of model-based OPC mask pattern data may be created. An appropriate verification method needs to be used for each of the rule-based OPC mask pattern data and the model-based OPC mask pattern data.